Abrasive Waterjet (AWJ) Forces—Indicator of Cutting System Malfunction

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Article Abrasive Waterjet (AWJ) Forces—Indicator of Cutting System Malfunction

Libor M. Hlaváˇc 1,* , Damian Ba ´nkowski 2 , Daniel Krajcarz 2, Adam Štefek 1, Martin Tyˇc 1 and Piotr Młynarczyk 2

1 Department of Physics, Faculty of Electrical Engineering and Computer Science, VSB–Technical University of Ostrava, 17. Listopadu 2172/15, 70800 Ostrava-Poruba, Czech Republic; [email protected] (A.Š.); [email protected] (M.T.) 2 Department of Materials Science and Materials Technology, Faculty of Mechatronics and Mechanical Engineering, Kielce University of Technology, al. Tysi ˛acleciaPa´nstwaPolskiego 7, 25-314 Kielce, Poland; [email protected] (D.B.); [email protected] (D.K.); [email protected] (P.M.) * Correspondence: [email protected]; Tel.: +420-59-732-3102

Abstract: Measurements enabling the online monitoring of the abrasive waterjet (AWJ) cutting process are still under development. This paper presents an experimental method which can be applicable for the evaluation of the AWJ cutting quality through the measurement of forces during the cutting process. The force measuring device developed and patented by our team has been used for measurement on several metal materials. The results show the dependence of the cutting to deformation force ratio on the relative traverse speed. Thus, the force data may help with a better understanding the interaction between the abrasive jet and the material, simultaneously impacting the improvement of both the theoretical and empirical models. The advanced models could substantially improve the selection of suitable parameters for AWJ cutting, milling or turning with the desired quality of product at the end of the process. Nevertheless, it is also presented that force Citation: Hlaváˇc,L.M.; Ba´nkowski, D.; Krajcarz, D.; Štefek, A.; Tyˇc,M.; measurements may detect some undesired effects, e.g., not fully penetrated material and/or some Młynarczyk, P. Abrasive Waterjet product distortions. In the case of a proper designing of the measuring device, the force measurement (AWJ) Forces—Indicator of Cutting can be applied in the online monitoring of the cutting process and its continuous control. System Malfunction. Materials 2021, 14, 1683. https://doi.org/10.3390/ Keywords: metals; abrasive waterjet; force measurement; cutting; force ratio ma14071683

Academic Editor: Victor Songmene 1. Introduction Received: 13 February 2021 The machining tool “abrasive waterjet” (AWJ) has been used as a modern machining Accepted: 24 March 2021 technology for about 40 years. It is either applied or tested for use in cutting [1], milling [2], Published: 29 March 2021 turning [3], piercing [4], grinding [5] or polishing [6]. AWJ is capable of machining materials independently of their thickness, provided that sufficient energy is supplied [7]. The Publisher’s Note: MDPI stays neutral regression or theoretical models are utilized to determine accuracy of this tool. No complex with regard to jurisdictional claims in model applicable for all types of materials and machining parameters has been prepared published maps and institutional affil- yet. Therefore, models used in practice are suitable for either ductile materials or for brittle iations. ones. A better understanding of the erosion process is essential for the improvement of the contemporary models or the preparation of some new and better ones. It can even aim for the creation of some new design of the AWJ tools. Many online and offline measurements have been made in past to gain information about quality of the AWJ machining process. Copyright: © 2021 by the authors. However, conventional online methods are often inappropriate, because the environment Licensee MDPI, Basel, Switzerland. is very unsuitable for electrical equipment, optics and other common sensors during the This article is an open access article machining process. The flexibility of the AWJ makes it strongly sensitive on all irregularities distributed under the terms and in jet flow and/or material properties. Rebounding of the abrasive water jet from the conditions of the Creative Commons Attribution (CC BY) license (https:// material or deflection of the jet into an unexpected direction increases the risk of tracking creativecommons.org/licenses/by/ technique damage. For the further expansion of AWJ applications in industry, there needs to 4.0/).

Materials 2021, 14, 1683. https://doi.org/10.3390/ma14071683 https://www.mdpi.com/journal/materials Materials 2021, 14, 1683 2 of 16

be an improvement of measuring processes [8]. Therefore, some new methods suitable for monitoring of “new technologies” have been studied and tested, as it was mentioned in [9]. Some new measuring methods were tested for the monitoring of the abrasive waterjet in milling, turning and 3D machining. The investigation of the acoustic emissions [10,11] is one of such unconventional methods used for AWJ measuring. Several attempts aimed at determining the quality of the machining process through the measurement of the cutting head vibrations were presented few years ago [12–14]. Vibrations of the machined material are often studied to find some new method applicable for AWJ process monitoring [15–17]. Some of the scientists have found, however, that more than 25% of the information from these sources may be incorrectly explained [18] due to misinterpretation of signal characteristics. These signals (acoustic emissions, cutting head vibrations, material vibrations) may be strongly influenced by the dimensions of the machine, its stiffness or the dimensions of the machined material. Force sensors were used in the past, namely for the determination either of the velocity profile [19,20] of the pure waterjet or some measurements of the waterjet and abrasive waterjet diameter [19–21]. The force measurements were also used for the investigation of pulsating jets [22]. The first application of force measurements on AWJ force measurements has been described in [23]. Another presented force measurement investigation was aimed at studying the mixing processes [24]. The more recent use of waterjet force measurements is focussed on hand tools [25], impact force at high pressures [26] and the impact of the waterjet structure on forces [27,28]. AWJ force measurements are once again coming to the fore [29,30], especially in connection with applications on new materials. The force measurements described in this contribution to the knowledge of the AWJ are focused on the static force component, eliminating the dynamic component with questionable origins. Supressing or even the elimination of the measured signal distortion caused by the effects mentioned above was the most important motivation for this study aimed at force measurements on samples with equal dimensions. The applied force sensor measuring x-y-z forces during the AWJ cutting process was designed and manufactured by our research group in 2011 and patented in 2012 [31]. Since that time, it has been used for the investigation of the impact of the AWJ on materials. The first remarks about force measurements performed with our sensor were presented at a conference in 2019 [32]. Research of the AWJ force’s dependence on the material structure was presented soon after [33]. Some recent findings are presented here, e.g., the application of the sensor for testing of the assumption regarding linkage between the ratio of forces in x and z orthogonal axes and the ratio of cutting to deformation wear (i.e., respective tangential and normal forces). The set of force measurements performed on metal samples is presented, evaluated and discussed in this article. As traverse speed is one of the key factors affecting force values, the different traverse speeds were applied testing the basic assumptions. Many similar experiments were performed, and some assumptions were verified. However, the results showing some specific and unusual features were also selected for presentation and discussion in this article primarily.

2. Theoretical Background One of the theoretical models presented by Hlaváˇc[34] is based on the cutting quality determination through the ratio of traverse speed to the limit one. Nowadays, the declination angle of striations measured at the bottom edge of a sample wall is used for cutting quality evaluation, as was presented in [35,36]. The speciﬁc factors introduced in these previous articles are namely the limit traverse speed and the limit declination angle. These factors can be utilized, as was presented in previous publications, for the determination of cutting characteristics ensuring the required quality of the cutting wall. As the traverse speed of the cutting head is the most easily changeable cutting factor, it is the most usual process variable used for quality control of the AWJ machining. Therefore, the most typical task of the AWJ operator is to set up an appropriate traverse speed, ensuring the desired quality of a product. An example of complex knowledge application, based on Materials 2021, 14, 1683 3 of 16

the theoretical description of an AWJ cutting through the declination angle and the taper, was demonstrated in [37], comparing results of the theoretical model and experimental investigation on columns cut from various metal samples. The simpliﬁed Hlaváˇc’smodel for limit traverse speed calculation can be expressed as Equation (1):

2 q 3  3 −5ξj L 2  C do 2ρj pj e (1 − αe ) vPlim =   − vPmin (1) 2 −2ξj L 8 H pjρmαe e + σmρj

The meaning of the variables in Equation (1) are as follows: vPlim: the limit traverse speed; C: coefficient including influence of abrasive mass flow rate, abrasive material quality and Ludolf’s number (for circular jet shapes); do: diameter of the water nozzle (orifice); ρj: density of the abrasive jet (conversion to a homogeneous liquid); pj: pressure obtained from Bernoulli’s equation for liquid with density and velocity of abrasive jet; ξj: attenuation coefficient of the abrasive jet in the environment between the focusing tube outlet and the material surface; L: stand-off distance (distance between the focusing tube outlet and the material surface); αe: coefficient of the abrasive water jet velocity loss in the interaction with the material (experimentally determined from a testing cut on a sample of the respective material); H: material thickness; ρm: density of the material being machined; σm: strength of the material being machined; vPmin: minimum limit traverse speed of cutting-correction for the zero traverse speed (usually the value vPmin = an/60 is used, where an is the average abrasive particle size after the mixing process inside the mixing head and focussing tube). The calculation of the limit traverse speed from the theoretical model is limited by properly determining all necessary inputs. However, some of these inputs are rarely known and, sometimes, are even hardly determinable. Therefore, Equation (2), being just a modification of an equation describing the relation between traverse speed and declination angle presented in [34], is often used:

2 ϑ 3 v = v lim (2) Plim P ϑ

where vP is the traverse speed value for which the declination angle ϑ is measured on the kerf wall; the limit traverse speed vPlim and the limit declination angle ϑlim are the limit values experimentally determined for selected material and cutting conditions. The limit declination angle ϑlim changes with the machine power and material resis- tance to machining by the AWJ. For sufﬁcient power and material thickness up to 50 mm, the angle is approximately 45◦. Increasing the material thickness or decreasing the AWJ power causes the deterioration of conditions necessary for the full development of the deformation removal mode in the material. The limit declination angle decreases to the value of 15◦, valid for a single cutting wear mode inside very thick materials (or for powerless machines). The opposite situation is when the material is very thin—such a material could be easily perforated due to the deformation mode and the limit declination angle seems to be over 50◦. The best sample thickness used for force measurement investigation seems to be within 5 and 25 mm. The samples thicker than 25 mm are usually heavy and the force sensor range in the impact force is reduced through loading by the gravity force caused by the sample mass. The theoretical knowledge presented in Equations (1) and (2) can be easily used for the elimination of the time-consuming experimental determination of limit traverse speeds for materials for which such a value has been determined on a certain machine. The equation for limit traverse speed recalculation can be derived namely in cases, when only a few Materials 2021, 14, 1683 4 of 16

speeds for materials for which such a value has been determined on a certain machine. The equation for limit traverse speed recalculation can be derived namely in cases, when only a few parameters are changed, e.g., pressure, nozzle diameter and abrasive mass flow rate. This method was used in experimental work presented in this article, determining Equation (3):

dp3 =η 22 vvPlim221 A3 Plim 1 (3) Materials 2021, 14, 1683 4 of 16 dp11

Variable vPlim2 is the limit traverse speed calculated for machine configuration 2; parametersv is the are changed,limit traverse e.g., pressure, speed nozzle already diameter known and abrasive for machine mass flow configuration rate. This 1; η is methodPlim1 was used in experimental work presented in this article, determining Equation (3): A21 a ratio of abrasive qualities (the seconds to the first one), if they differ; dd12, are the d p3 = 2 2 respective nozzle diametersvPlim2 ofη A21configurations;3 vPlim1 p1,p 2 are the respective(3) pumping d1 p1 pressures of configurations. VariableThis importantvPlim2 is the limit result traverse derived speed calculatedfrom th fore machinetheoretical configuration model 2; vhasPlim 1 been used for is the limit traverse speed already known for machine configuration 1; η is a ratio of experimental planning, because the original limit values wereA 21determined for parameters abrasive qualities (the second to the first one), if they differ; d1, d2 are the respective nozzle diametersof the machine of configurations; at the VSB—Technicalp1, p2 are the respective Universi pumpingty of pressures Ostrava of configurations.(Ostrava, Czech Republic), but Thismeasurements important result were derived perfor frommed the on theoretical machine model with has differen been usedt parameters for experi- at the Kielce mentalUniversity planning, of Technology because the original (Kielce, limit Poland). values were determined for parameters of the machine at the VSB—Technical University of Ostrava (Ostrava, Czech Republic), but mea- The traverse speed of the cutting head is the most appropriate variable for studies of surements were performed on machine with different parameters at the Kielce University ofrelations Technology between (Kielce, Poland).the force measurement and the cutting process quality, as it can be easilyThe changed traverse speed and ofvery the cuttingprecisely head set is the up. most The appropriate cutting (tangential variable for studies to the of sample surface) relationsand the between deformation the force (normal measurement to the and sample the cutting surface) process quality,forces aswere it can defined be easily as indicated in changedFigure and1. very precisely set up. The cutting (tangential to the sample surface) and the deformation (normal to the sample surface) forces were defined as indicated in Figure1.

FigureFigure 1. 1.Scheme Scheme of the of force the decompositionforce decomposition in a certain in point a cert onain the headingpoint on line the characterizing heading line the characterizing trajectorythe trajectory of the penetrating of the penetrating jet (the perpendicular jet (the perpendicu direction tolar the direction traverse speed to the and traverse gravity is speed not and gravity is analysednot analysed in this study,in this so study, the impinging so the forceimpinging is reduced force into is two reduced dimensions). into two dimensions). The consideration of the sketched force decomposition (Figure1) leads to the presumption postulatingThe consideration this statement: of “The the cutting sketched to deformation force decomposition force ratio (CDFR) (Figure depends 1) leads to the onpresumption the ratio of the postulating cutting-to-deformation this statement: wear inside “The the cutting produced to kerf”.deformation Based on force the ratio (CDFR) experimentaldepends on results, the ratio it is assumedof the cutting-to-deformat that increasing the traverseion wear speed inside from the the zero produced value kerf”. Based induceson the theexperimental respective increase results, in the it is CDFR. assumed The maximum that increasing of the CDFR the is traverse expected speed to be from the zero around half of the limit traverse speed. Increasing the traverse speed above the value corre- spondingvalue induces to the maximum the respective of the CDFR increase leads to the in decrease the CDFR. in the CDFRThe value.maximum However, of the CDFR is thisexpected consideration to be is around valid only half in cases of the when limit the energy traverse of the speed. jet is sufficient Increasing for the the full traverse speed developmentabove the value of both corresponding types of wear. If the to jetthe energy maximum is lower of than the necessary CDFR leads (when cuttingto the decrease in the veryCDFR thick value. materials), However, only the this cutting consideration wear provides is penetration valid only through in cases the material.when the The energy of the jet residual jet energy inside the deep kerfs is insufficient for the proper development of the deformationis sufficient wear. for the Therefore, full development when the conditions of both (namely types traverse of wear. speed) If the are jet changed energy is lower than sonecessary that cutting (when force itselfcutting is insufficient very thick for materials), material removal, only the the residual cutting jet wear reflects provides back penetration fromthrough the kerf. the This material. situation The is typical residual for an jet outlet energy declination inside angle the deep overcoming kerfs ais value insufficient for the ◦ ofproper about 22.5developmentfor medium-thick of the materials deformation (1.5–2.5 wear. thicker Therefore, than is appropriate when forthe the conditions jet (namely energy) or 15◦ for very thick materials (more than 2.5 times thicker than is appropriate for

Materials 2021, 14, 1683 5 of 16

traverse speed) are changed so that cutting force itself is insufficient for material removal, Materials 2021, 14, 1683 the residual jet reflects back from the kerf. This situation is typical for an5 of 16outlet declination angle overcoming a value of about 22.5° for medium-thick materials (1.5–2.5 thicker than is appropriate for the jet energy) or 15° for very thick materials (more than 2.5the jettimes energy). thicker A similarthan is situationappropriate can alsofor the be causedjet energy). by the A decrease similar insituation the AWJ can power also be (insufﬁcientcaused by the pumping decrease system in the or AWJ pump power failure). (insufficient pumping system or pump failure).

3. Measuring SystemSystem Description Description A devicedevice measuringmeasuring forces forces in in three three orthogonal orthogon axesal axes has has been been developed, developed, constructed constructed and patented byby our our team team [31 [31].]. The The design design is is shown shown in Figurein Figure2. 2.

FigureFigure 2. 2. ForceForce sensor sensor designed andand constructedconstructed for for abrasive abrasive waterjet waterjet (AWJ) (AWJ) force force measurements measurements (ampliﬁers (amplifiers for strainfor strain gaugesgauges are are closed closed in black boxesboxes beingbeing blown blown through through by by pressurized pressurized air air to preventto prevent moisture). moisture).

The measuringmeasuring system system consists consists of twelveof twelve deformation deformation elements, elements, eachone each covered one covered by bytwo two extensometers. extensometers. The The six six electric electric signals signal ares are amplified amplified by amplifiers by amplifiers closed closed in plastic in plastic boxes (Figure(Figure2 ),2), each each one one for for a fulla full Wheatstone Wheatstone bridge bridge composed composed of four of four extensometers extensometers on two deformationdeformation elements elements in in the the appropriate appropriate direction. direction. The The amplifiers amplifiers are suppliedare supplied by by an external directdirect currentcurrent source source 15 15 V. V. The The amplified amplified signal signal enters enters the the analog-to-digital analog-to-digital transducer andand hencehence a a computer. computer. Recording Recording and and processing processing of theof the signal signal is performed is performed using the software Signal Express (2012) and LabVIEW (2012), both from the National using the software Signal Express (2012) and LabVIEW (2012), both from the National Instruments Corp., Austin, TX, USA. All six signals are recorded via a program prepared Instruments Corp., Austin, TX, USA. All six signals are recorded via a program prepared in the Signal Express, saving the time needed for individual measurements. The raw in the Signal Express, saving the time needed for individual measurements. The raw voltage signals are processed in the LabVIEW program in any subsequent unlimited time. Thisvoltage handling signals of are the signalprocessed is used in duringthe LabVIEW the research program state. in However, any subsequent the continuous unlimited time.evaluation This necessaryhandling forof onlinethe signal control is canused be easilyduring obtained the research by modifying state. However, either the the continuousSignal Express evaluation program ornecessary the LabVIEW for online one. control can be easily obtained by modifying either the Signal Express program or the LabVIEW one. 4. Experimental Setup, Presumptions and Results 4. ExperimentalAll presented Setup, measurements Presumptions were and performed Results in the Faculty of Mechatronics and MechanicalAll presented Engineering, measurements Department were of Materials performed Science in andthe MaterialsFaculty of Technology Mechatronics at the and MechanicalKielce University Engineering, of Technology, Department Kielce, Poland.of Mate Thisrials internationalScience and cooperationMaterials Technology was used at theto test Kielce forces University on a less powerful of Technology, and smaller Kielce, cutting Poland. machine This than international the one used cooperation in Ostrava was (VSB—Technical University of Ostrava, Ostrava, Czech Republic). The experimental factors and applied settings of the abrasive waterjet are shown in Table1.

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Table 1. Summary of factors and settings in experiments.

Variable (Unit) Value Pump pressure (MPa) 250 Water oriﬁce diameter (mm) 0.33 Focusing tube diameter (mm) 1.02 Focusing tube length (mm) 76 Abrasive mass ﬂow rate (g/min) 240 Abrasive material average grain size (mm) 0.177 Abrasive material type Indian garnet Stand-off distance (mm) 2 Traverse speed (mm/min) 50–400

Several squared metal samples presented in Tables2 and3 were used for the experiments. Their uniaxial tensile strength (σm), density (ρm) and Vickers hardness (HV10) are summarized in Table2. All presented samples were plates with dimensions of 120 × 60 × T in mm, where T is thickness. The linear cuts perpendicular to the longer side were performed around the middle of this side and they extended approximately 20 mm into the sample. The samples were ﬁxed to the base plate of the sensor by two steel strips pressed onto the material by nuts on screws approximately in 1/6 and 5/6 of the sample length. The base plate with dimensions of 170 × 170 × 3 in mm is made of low alloy high strength steel and it has a central circular hole with a diameter of 80 mm. This base plate is input to the frame named “sample ﬁxture” in Figure2 and fastened with a quick release. The tested samples were positioned so that the entire path of the abrasive jet lay inside the hole of the base plate. The unsupported area of the sample (due to the hole in the base plate) did not exceed 40% of the sample area. The jet path started approximately 3 mm outside the material edge. All cuts were performed in an x-axis direction. Therefore, y-axis forces (side forces in kerfs) were negligible compared to those in x and z directions. These signals caused by cutting were of the order 10−2 V in y direction, while signals in x and z directions were of the order of volts. Therefore, the signal from the y direction was not included in the analyses.

Table 2. Materials used in experiments (10 mm thick). σm: uniaxial tensile strength; ρm: density; HV10: Vickers hardness.

σm ρm WRN (DIN) Norm HV10 (MPa) (kg·m−3) 1.0547 (St 52-3) construction steel 445 7772 125 1.0503 (C 45) low alloy steel 621 7651 170 1.7131 (16 MnCr 5) high strength steel 880 7746 252 1.4541 (X6 CrNiTi 18 10) stainless steel 515 7521 166 Hardox 500 special low abrasion steel 1679 7524 544 3.1325 (duralumin 2017) duralumin 419 2784 122 2.0060 (E-Cu58) copper 211 8687 40 brass 2.0402 (CuZn40Pb2) brass 393 8364 61

Table 3. Samples from 34CrMo4 steel used for the production of pressure cylinders: austenitized, quenched and tempered.

Sample Mark Tempering Uniaxial Strength Sample Mark Tempering Uniaxial Strength (10 mm Thick) Temperature (◦C) (MPa) (6 mm Thick) Temperature (◦C) (MPa) K37 20 2230 K43 20 2160 K38 250 1865 K44 250 1860 K39 400 1560 K45 400 1550 K40 510 1340 K46 510 1320 K41 580 1190 K47 580 1230 K42 620 1060 K48 640 970 Materials 2021, 14, 1683 7 of 16

Beside the basic metal set also used in past experiments [37], two additional sets of samples were prepared of 34CrMo4 steel (DIN norm), each one with a different nickel ratio. These samples were austenitized at 850 ◦C, quenched in polymer and tempered at various temperatures; their tempering and respective uniaxial strengths are presented in Table3. Samples prepared of the mentioned two steel modifications with identical heat treatment were also used for research aimed at the steel structure impact on AWJ machining presented in [38]. The sampling frequency used for force signal records was 20 kHz. This frequency yields the possibility of studying characteristic frequencies of signals up to 10 kHz, but there are rarely any significant frequencies higher than 5 kHz in our recorded signals. In order to prove the presumption that increasing the traverse speed leads to the increase in the CDFR up to maximum value with a subsequent decreasing, three cuts were carried out on each one sample. Making a particular cut, the traverse speed of the cutting head was increased. Different speed sets were used for individual material types to retain a similar traverse speed ratio regarding the limit traverse speed for each tested material. The lowest traverse speeds correspond with a very high quality of cutting, producing small declination angles of striations on the cut walls—about 6◦ (up to 10◦). The medium traverse speeds correspond with often used quality in practice being a compromise between production speed and economy—the declination angles of striations on the cut walls are between 15◦ and 25◦. The highest traverse speeds correspond with a poor quality of cutting ◦ Materials 2021, 14for, 1683 large declination angles of striations on the cut walls, typically over 30 . The force 8 of 16 sensor is quite sensitive to accidental excitements caused, for example, by imperfections of the cutting table. Therefore, it was impossible to arrange the same conditions for all measurements performed on different days or after a break due to different work on the assumed to be the force value in the respective direction (axis). The principle is the same equipment. However, the observed shifts in absolute values (caused by different fixing of in both considered axes representing the cutting and deformation force (x and z axis, the measuring devicerespectively). or samples) The ratio are ofof thethe correspondingx and z force values size inis alldetermined axes. Therefore, directly thein the prepared cutting to deformationLabVIEW force program ratio (ratio for ofeach average material, values for in wh axesich x the and relevant z; see Figure records2) was were obtained. analysed. The typicalSignals time from dependent both axes force are measured signal record simult is presentedaneously. inThe Figure set of3. force It shows ratio values and the almost idealrespective shape of forcerelative signal traverse in the sp zeeds axis. is summarized in Table 4.

Figure 3. ShapeFigure of the3. Shape typical of measuredthe typical signal measured in the signal z axis inwith the zdescription. axis with description.

Table 4. The ratioThe of processing the x and z forces procedure (cutting starts and deformation) with the transformation measured by force of thesensor voltage with different signal to traverse the speed sets (boldforce signed signal values), through the respective calibration limit traverse equations speeds prepared are in a separate for each column couple and ofthe deformation relative traverse ele- speeds are presented in brackets [ ]. ments representing one Wheatstone bridge (calibration was performed in a static mode;

Sample Thicknessthe increasing (10 mm) and decreasingLimit vP values force was realized50 mm/min by adding or 100 removing mm/min weights). 150 Si- mm/min 1.0547multaneously, (St 52-3) the signal level 194 at the beginning0.52 [0.26] of the measurement0.71 [0.52] is moved to a zero0.55 [0.77] 1.7131level (16 accordingMnCr 5) to the auto-calibration 170 signal0.49 sample [0.29] selected from0.73 the [0.59] signal without the0.54 [0.88] 1.0503switched (C 45) waterjet machine. Both 155 of these steps0.56 are [0.32] performed in the0.73 program [0.65] automatically0.45 [0.97] 1.4541 (X6 CrNiTi 18 10) 155 0.53 [0.32] 0.70 [0.65] 0.48 [0.97] Sample Thickness (10 mm) 100 mm/min 150 mm/min 200 mm/min 2.0060 (E-Cu 58) 254 0.56 [0.39] 0.62 [0.59] 0.58 [0.79] brass 2.0360 (CuZn40) 252 0.62 [0.40] 0.73 [0.60] 0.55 [0.79] Sample Thickness (10 mm) 50 mm/min 75 mm/min 100mm/min 34CrMo4/* TT 20 °C 121 0.50 [0.41] 0.60 [0.62] 0.51 [0.83] 34CrMo4/TT 250 °C 129 0.60 [0.39] 0.70 [0.58] 0.61 [0.78] 34CrMo4/TT 400 °C 136 0.60 [0.37] 0.69 [0.55] 0.57 [0.73] 34CrMo4/TT 510 °C 141 0.50 [0.35] 0.61 [0.53] 0.52 [0.71] 34CrMo4/TT 580 °C 148 0.52 [0.34] 0.66 [0.51] 0.58 [0.67] 34CrMo4/TT 620 °C 157 0.50 [0.32] 0.65 [0.48] 0.59 [0.64] Sample Thickness (6mm) 100 mm/min 125 mm/min 150 mm/min 34CrMo4/TT 20 °C 186 0.60 [0.53] 0.68 [0.67] 0.56 [0.81] 34CrMo4/TT 250 °C 204 0.54 [0.48] 0.67 [0.61] 0.61 [0.73] 34CrMo4/TT 400 °C 221 0.59 [0.45] 0.66 [0.57] 0.57 [0.68] 34CrMo4/TT 510 °C 227 0.53 [0.44] 0.67 [0.55] 0.57 [0.66] 34CrMo4/TT 580 °C 228 0.51 [0.43] 0.65 [0.54] 0.56 [0.65] 34CrMo4/TT 640 °C 253 0.50 [0.40] 0.60 [0.49] 0.57 [0.59] * TT—tempering temperature.

Materials 2021, 14, 1683 8 of 16

for each of the measured signals, but the operator can change the selection of the signal segment used for calibration. The basic task during signal processing is to deﬁne the appropriate part of the signal after triggering the AWJ to set the lowest level of the force and to select the “stable” part of the signal during AWJ cutting of the material for setting the higher level of the force. Both parts of the signal are selected manually according to the respective timeline (Figure3). The median values of each level are counted and subtracted from each other. The difference of these median values is assumed to be the force value in the respective direction (axis). The principle is the same in both considered axes representing the cutting and deformation force (x and z axis, respectively). The ratio of the x and z force values is determined directly in the prepared LabVIEW program for each material, for which the relevant records were obtained. Signals from both axes are measured simultaneously. The set of force ratio values and respective relative traverse speeds is summarized in Table4.

Table 4. The ratio of the x and z forces (cutting and deformation) measured by force sensor with different traverse speed sets (bold signed values), the respective limit traverse speeds are in a separate column and the relative traverse speeds are presented in brackets [ ].

Sample Thickness (10 mm) Limit vP Values 50 mm/min 100 mm/min 150 mm/min 1.0547 (St 52-3) 194 0.52 [0.26] 0.71 [0.52] 0.55 [0.77] 1.7131 (16 MnCr 5) 170 0.49 [0.29] 0.73 [0.59] 0.54 [0.88] 1.0503 (C 45) 155 0.56 [0.32] 0.73 [0.65] 0.45 [0.97] 1.4541 (X6 CrNiTi 18 10) 155 0.53 [0.32] 0.70 [0.65] 0.48 [0.97] Sample Thickness (10 mm) 100 mm/min 150 mm/min 200 mm/min 2.0060 (E-Cu 58) 254 0.56 [0.39] 0.62 [0.59] 0.58 [0.79] brass 2.0360 (CuZn40) 252 0.62 [0.40] 0.73 [0.60] 0.55 [0.79] Sample Thickness (10 mm) 50 mm/min 75 mm/min 100 mm/min 34CrMo4/* TT 20 ◦C 121 0.50 [0.41] 0.60 [0.62] 0.51 [0.83] 34CrMo4/TT 250 ◦C 129 0.60 [0.39] 0.70 [0.58] 0.61 [0.78] 34CrMo4/TT 400 ◦C 136 0.60 [0.37] 0.69 [0.55] 0.57 [0.73] 34CrMo4/TT 510 ◦C 141 0.50 [0.35] 0.61 [0.53] 0.52 [0.71] 34CrMo4/TT 580 ◦C 148 0.52 [0.34] 0.66 [0.51] 0.58 [0.67] 34CrMo4/TT 620 ◦C 157 0.50 [0.32] 0.65 [0.48] 0.59 [0.64] Sample Thickness (6 mm) 100 mm/min 125 mm/min 150 mm/min 34CrMo4/TT 20 ◦C 186 0.60 [0.53] 0.68 [0.67] 0.56 [0.81] 34CrMo4/TT 250 ◦C 204 0.54 [0.48] 0.67 [0.61] 0.61 [0.73] 34CrMo4/TT 400 ◦C 221 0.59 [0.45] 0.66 [0.57] 0.57 [0.68] 34CrMo4/TT 510 ◦C 227 0.53 [0.44] 0.67 [0.55] 0.57 [0.66] 34CrMo4/TT 580 ◦C 228 0.51 [0.43] 0.65 [0.54] 0.56 [0.65] 34CrMo4/TT 640 ◦C 253 0.50 [0.40] 0.60 [0.49] 0.57 [0.59] * TT—tempering temperature.

5. Discussion of Results All measurements indicate the fact that with increasing traverse speed, the CDFR increases, firstly up to a certain maximum value and then decreases. This result can also be seen in Figure4, showing the CDFR dependence on the relative traverse speed. Therefore, it can be said that the measurements prove this presumption well. The principle of the measuring device is based on the deformation of the metal deformation elements. However, these deformation elements are not infinitely tough, and the calibration equations do not include the hysteresis behaviour fully (it is also dependent on the fixing of the device, sample weight, etc.). The irremovable hysteresis makes it impossible to compare absolute values of measurements performed at different times and in different conditions. Opposite to this, the ratio of the forces (deformation to cutting force) seems to be independent on the fixing conditions and changes, because shifts caused by the fixing of the sample weight influence all axes (directions of measurement) proportionally. The calculated Materials 2021, 14, 1683 9 of 16

5. Discussion of Results All measurements indicate the fact that with increasing traverse speed, the CDFR increases, firstly up to a certain maximum value and then decreases. This result can also be seen in Figure 4, showing the CDFR dependence on the relative traverse speed. Therefore, it can be said that the measurements prove this presumption well. The principle of the measuring device is based on the deformation of the metal deformation elements. However, these deformation elements are not infinitely tough, and the calibration equations do not include the hysteresis behaviour fully (it is also dependent on the fixing of the device, sample weight, etc.). The irremovable hysteresis makes it impossible to compare absolute values of measurements performed at different times and in different conditions. Opposite to this, the ratio of the forces (deformation to Materials 2021, 14, 1683 cutting force) seems to be independent on the fixing conditions and changes,9 of 16 because shifts caused by the fixing of the sample weight influence all axes (directions of measurement) proportionally. The calculated combined uncertainty of measurements is combinedapproximately uncertainty 8%, ofincluding measurements the uncertainty is approximately of the 8%, device including measuring the uncertainty electric signal, ofvariability the device of measuring material electricproperties signal, and variability stability of materialcutting parameters. properties and Therefore, stability of the CDFR cuttingis a suitable parameters. quantity Therefore, for comparison. the CDFR is a suitable quantity for comparison.

FigureFigure 4. 4.Relationship Relationship between between the cuttingthe cutting to deformation to deform forceation ratio force (CDFR) ratio values(CDFR) and values the relative and the traverserelative speed traverse values. speed values.

TheThe force force value value is dependentis dependent on the on deﬁned the defined signal si ongnal the on timeline. the timeline. Selected Selected sections sections mightmight cause cause different different counted counted force force values. values. To eliminate To eliminate this potential this potential distortion, distortion, the the median values of each level are counted and each of the selected timeline levels should median values of each level are counted and each of the selected timeline levels should be be as long as possible. The signal level median values are resistant to outliers caused by improperas long selection.as possible. The The “theoretical” signal level curve median in Figure 4values was determined are resistant on the to assumptionoutliers caused by thatimproper the dependence selection. of theThe force “theoretical” ratio on the cu relativerve in traverse Figure speed 4 was is parabolic. determined The on the necessaryassumption constants that the for thedependence mathematical of expressionthe force ofratio the curveon the were relative determined traverse from speed is theparabolic. regression The of thenecessary results. The constants resulting for equation the ma usedthematical for the calculation expression of the of curvethe curve in were thedetermined graph presented from inthe Figure regression4 as “theory” of the has thisresu form:lts. The resulting equation used for the

calculation of the curve in the graph presented2 in Figure 4 as “theory” has this form: CDFR = −1.8vr + 2.2vr − 0.02 (4) =−2 + − CDFR 18...vvrr 22 002 (4) The ratio of actual traverse speed vP to the limit one vPlim, i.e. (vP/vPlim), is signed vr (relative traverse speed) in Equation (4). v v vv TheThe problem ratio of isactual a limited traverse number speed of possibleP to the measurements. limit one Plim The, i.e. number ( P Plim is not), is signed sufficientvr (relative for quality traverse statistical speed) conclusions. in Equation Moreover, (4). some of the force ratio values can be influenced by changed cutting conditions of similar measurements performed earlier, such as another cutting table, another pumping device, etc. All these changes influence the stiffness of the respective cutting device and the corresponding fixing possibilities. Beside these sources of systematic uncertainties shifting signals in all axes, some other irregularities occurred in some of the measured signals. Several unusual signals are presented in Figures5–8 and are commented on in further text. They may bring some new information about AWJ cutting. The sources of the unusualness are the subject of the contemporary and future research. Figure5 shows a significant shift during AWJ cutting. This shift may be justified in higher levels of the water in the AWJ residual energy attenuator. Splashing water can induce additional deformation of the deformation elements and suddenly shift the measuring level. Generally, this seems to be caused by a quick mechanical impact on the Materials 2021, 14, 1683 10 of 16

The problem is a limited number of possible measurements. The number is not sufficient for quality statistical conclusions. Moreover, some of the force ratio values can be influenced by changed cutting conditions of similar measurements performed earlier, such as another cutting table, another pumping device, etc. All these changes influence the stiffness of the respective cutting device and the corresponding fixing possibilities. Beside these sources of systematic uncertainties shifting signals in all axes, some other irregularities occurred in some of the measured signals. Several unusual signals are presented in Figures 5–8 and are commented on in further text. They may bring some new information about AWJ cutting. The sources of the unusualness are the subject of the contemporary and future research. Figure 5 shows a significant shift during AWJ cutting. This shift may be justified in higher levels of the water in the AWJ residual energy attenuator. Splashing water can induce additional deformation of the deformation elements and suddenly shift the Materials 2021, 14measuring, 1683 level. Generally, this seems to be caused by a quick mechanical impact on the10 of 16 AWJ cutting process. Hysteresis is well visible when comparing the beginning and the final levels of the signal. Nevertheless, two almost identical values can be evaluated taking the first force “step”AWJ between cutting the process. level Hysteresis of signal is from well visible8 to 13 when s and comparing the one thefrom beginning 15 to 23 and s and the ﬁnal the second forcelevels “step” of the calculated signal. Nevertheless, from signals two taken almost from identical timespans values canfrom be evaluated24 to 30 s taking and the from 32 to 37 s.ﬁrst Analogically, force “step” Figure between 6 theshows level so ofme signal kind from of interruption 8 to 13 s and the at onethe frombeginning 15 to 23 of s and the measurementthe second(before force the “step” cut in calculated the mate fromrial). signals No takenforce fromvalue timespans can be from determined 24 to 30 s and directly. However,from comparing 32 to 37 s. Analogically, the signal Figurelevel from6 shows the some time kind interval of interruption from 17 atto the46 beginnings and of the measurement (before the cut in the material). No force value can be determined signal level in timespan from 49 to 51 s, certain force values can be calculated again, similar directly. However, comparing the signal level from the time interval from 17 to 46 s and to the case presentedsignal levelin Figure in timespan 5. Neverthele from 49 toss, 51 these s, certain “damaged” force values records can be calculatedwere not again,used similarin the analyses of tothe the presented case presented force in ratio Figure depe5. Nevertheless,ndence on these relative “damaged” traverse records speed. were They not are used in presented just asthe examples analyses ofof the the presented potential force problems ratio dependence of force measurements. on relative traverse speed. They are presented just as examples of the potential problems of force measurements.

Materials 2021, 14, 1683 11 of 16 Figure 5.FigureDeformation 5. Deformation force signal withforce a signal signiﬁcant with shift a significant during the stableshift during part of cutting the stable (sample part of of steel cutting 1.4541, (sample traverse speed 100of mm/min). steel 1.4541, traverse speed 100 mm/min).

Figure 6.FigureDeformation 6. Deformation force signal force with a signal signiﬁcant with shift a significant during the shift beginning during part the of cuttingbeginning (sample part Hardox of cutting 500 plate, traverse speed(sample 50 mm/min).Hardox 500 plate, traverse speed 50 mm/min).

Figures 7 and 8 show the signal, where the evident increase or decrease in the signal during the cutting stage (see part of the signal defined in Figure 3) is recorded. The process of this phenomenon is not as sudden as in the signal mentioned above. Some explanation could be the presence of supporting slats underneath the workpiece or improper abrasive delivery. Nevertheless, the measuring device was intentionally placed into positions at which no rebounding of the jet on supporting parts of the cutting table could take place. The delivery of the abrasive was continuously monitored, as well as the pumping pressure and the cutting device motion. All experiments with observed problems with the machine equipment were stopped and repeated. The presented results are observed in measurements with no visible problems except the case presented in the last figure of this article—the force record when the pump seal was broken.

Figure 7. Deformation force signal with presumed uncut parts—normal force increases on the uncut material parts (duralumin 2017, traverse speed 400 mm/min). Peaks (a, b, c, d) correspond with increase of deformation force on uncut parts (Figure 9), where jet rebounds.

Materials 2021, 14, 1683 11 of 16

Figure 6. Deformation force signal with a significant shift during the beginning part of cutting (sample Hardox 500 plate, traverse speed 50 mm/min).

Figures 7 and 8 show the signal, where the evident increase or decrease in the signal during the cutting stage (see part of the signal defined in Figure 3) is recorded. The process of this phenomenon is not as sudden as in the signal mentioned above. Some explanation could be the presence of supporting slats underneath the workpiece or improper abrasive delivery. Nevertheless, the measuring device was intentionally placed into positions at which no rebounding of the jet on supporting parts of the cutting table could take place. The delivery of the abrasive was continuously monitored, as well as the pumping pressure and the cutting device motion. All experiments with observed problems with the machine equipment were stopped and repeated. The presented results Materials 2021, 14are, 1683 observed in measurements with no visible problems except the case presented in the11 of 16 last figure of this article—the force record when the pump seal was broken.

Figure 7.FigureDeformation 7. Deformation force signal force with signal presumed with uncutpresumed parts—normal uncut parts—normal force increases force on the increases uncut material on the parts Materials 2021, 14, 1683 12 of 16 (duraluminuncut 2017, material traverse parts speed (duralumin 400 mm/min). 2017, Peaks traverse (a, b, c, sp d)eed correspond 400 mm/min). with increase Peaks of (a, deformation b, c, d) correspond force on uncut parts (Figurewith 9),increase where jet of rebounds. deformation force on uncut parts (Figure 9), where jet rebounds.

Figure 8.FigureCutting 8. force Cutting signal force with presumedsignal with uncut presumed parts—tangential uncut pa forcerts—tangential decreases on the force uncut decreases material parts on the (duralumin uncut 2017, traversematerial speed parts 400 mm/min).(duralumin Peaks 2017, (a, b,traverse c, d) indicate speed opposite 400 mm/min). direction ofPeaks cutting (a, force b, c, on d) the indicate uncut parts opposite (Figure 9) induceddirection at the end of them.cutting force on the uncut parts (Figure 9) induced at the end of them. Figures7 and8 show the signal, where the evident increase or decrease in the signal The explanationduring of the the cutting peaks stage marked (see part a, ofb, thec, d signal in signals deﬁned presented in Figure3 )in is Figures recorded. 7 The and process 8 is based on the ofphoto this phenomenonof the sample isnot after as suddenthe AWJ as cut in the (Figure signal mentioned9). There are above. several Some visible explanation points where thecould AWJ be does the presence not cut ofthrough supporting mate slatsrial. underneath They are themarked workpiece by identical or improper letters abrasive (a, b, c, d) as thedelivery. marks put Nevertheless, into graphs the of measuring both significant device wassignals intentionally (Figures placed7 and 8). into Peaks positions at which no rebounding of the jet on supporting parts of the cutting table could take marked (a, b, c, d) in Figure 7 correspond with increase of deformation force on uncut place. The delivery of the abrasive was continuously monitored, as well as the pumping parts, where jet rebounds. In Figure 8 the respective peaks (a, b, c, d) indicate opposite direction of cutting force on the uncut parts induced at the end of them. The zero level in x-axis is gradually lowered due to hysteresis of the measuring system.

Figure 9. Bottom side of the sample with marked uncut parts causing significant force changes (duralumin 2017, traverse speed 400 mm/min). The uncut parts of material marked a, b, c, d are corresponding with peaks marked with the same letters in the signal records in Figures 7 and 8.

The positions of the marks in the graphs were calculated from the jet traverse speed and the initial position of the cutting head regarding the material sample. The reflection of fluid flow on the uncut parts of material causes the rapid and short increase in the impact force up to twice the value of the cutting condition. This increase is consistent with the theory of liquid flow impact on a solid-state obstacle.

Materials 2021, 14, 1683 12 of 16

Figure 8. Cutting force signal with presumed uncut parts—tangential force decreases on the uncut material parts (duralumin 2017, traverse speed 400 mm/min). Peaks (a, b, c, d) indicate opposite direction of cutting force on the uncut parts (Figure 9) induced at the end of them.

The explanation of the peaks marked a, b, c, d in signals presented in Figures 7 and 8 Materials 2021is, 14 based, 1683 on the photo of the sample after the AWJ cut (Figure 9). There are several visible 12 of 16 points where the AWJ does not cut through material. They are marked by identical letters (a, b, c, d) as the marks put into graphs of both significant signals (Figures 7 and 8). Peaks marked (a, b, c, d)pressure in Figure and the7 correspond cutting device with motion. increase All experiments of deformation with observed force on problems uncut with the parts, where jet rebounds.machine equipment In Figure were 8 the stopped respective and repeated. peaks (a, The b, c, presented d) indicate results opposite are observed in direction of cuttingmeasurements force on the with uncut no visibleparts induced problems at except the end the caseof them. presented The inzero the level last ﬁgure in of this x-axis is graduallyarticle—the lowered due force to record hysteresis when theof the pump measuring seal was broken.system.

Figure 9. FigureBottom 9. Bottom side of theside sample of the with sample marked with uncut marked parts un causingcut parts significant causing force significant changes (duralumin force changes 2017, traverse speed 400 mm/min). The uncut parts of material marked a, b, c, d are corresponding with peaks marked with the same (duralumin 2017, traverse speed 400 mm/min). The uncut parts of material marked a, b, c, d are letters in the signal records in Figures7 and8. corresponding with peaks marked with the same letters in the signal records in Figures 7 and 8. The explanation of the peaks marked a, b, c, d in signals presented in Figures7 and8 The positionsis of based the onmarks the photo in the of graphs the sample were after calculated the AWJ cutfrom (Figure the jet9). traverse There are speed several visible and the initial positionpoints whereof the thecutting AWJ doeshead not regarding cut through the material. material They sample. are marked The reflection by identical letters of fluid flow on the(a, b, uncut c, d) as parts the marks of material put into graphscauses ofthe both rapid significant and short signals increase (Figures 7in and the8). Peaks impact force up tomarked twice (a, the b, c,value d) inFigure of the7 correspondcutting condition. with increase This of deformationincrease is forceconsistent on uncut parts, with the theory ofwhere liquid jet flow rebounds. impact In on Figure a soli8 thed-state respective obstacle. peaks (a, b, c, d) indicate opposite direction of cutting force on the uncut parts induced at the end of them. The zero level in x-axis is gradually lowered due to hysteresis of the measuring system. The positions of the marks in the graphs were calculated from the jet traverse speed and the initial position of the cutting head regarding the material sample. The reflection of fluid flow on the uncut parts of material causes the rapid and short increase in the impact force up to twice the value of the cutting condition. This increase is consistent with the theory of liquid flow impact on a solid-state obstacle. The malfunction of the pumping machine also occurred during measurements and it was recorded and evaluated. The resulting time dependent force signal can be seen in Figure 10. The impacting force copies strokes of the machine and the force peaks are much higher due to the reflection of the abrasive water flow from a fixed obstacle—the kerf bottom. The work of the pump significantly contributes to the peak formation, as can be seen in the signal shape. The pressure changes caused by the change of the piston movement direction in the multiplier can be seen on the record. The small peaks between the high ones are the strokes of the multiplier side with leaking seals. This side fails to achieve the necessary pressure. The highly fluctuating pressure level does not reach the required value for the proper suction of the abrasive material, causing the resulting increase in force to values several times higher than the ones observed for the abrasive jet with the proper pressure, because almost pure water is impinging the sensor. Materials 2021, 14, 1683 13 of 16

The malfunction of the pumping machine also occurred during measurements and it was recorded and evaluated. The resulting time dependent force signal can be seen in Figure 10. The impacting force copies strokes of the machine and the force peaks are much higher due to the reflection of the abrasive water flow from a fixed obstacle—the kerf bottom. The work of the pump significantly contributes to the peak formation, as can be seen in the signal shape. The pressure changes caused by the change of the piston movement direction in the multiplier can be seen on the record. The small peaks between the high ones are the strokes of the multiplier side with leaking seals. This side fails to achieve the necessary pressure. The highly fluctuating pressure level does not reach the required value for the proper suction of the abrasive material, causing the resulting Materials 2021, 14increase, 1683 in force to values several times higher than the ones observed for the abrasive jet13 of 16 with the proper pressure, because almost pure water is impinging the sensor.

Figure 10.FigureCutting 10. force Cutting signal force from cuttingsignal withfrom a cutti pumpng with with a damaged a pump seal with in onea damage side of thed seal multiplier; in one theside cut of material the was steelmultiplier; 1.0547 (St 52-3), the cut traverse material speed was 50 mm/min. steel 1.0547 (St 52-3), traverse speed 50 mm/min.

The record presentedThe record in presentedFigure 10 in was Figure ob tained10 was obtainedduring the during preparation the preparation of the of new the new set of experiments. When the problem of the pump was observed, a smaller orifice set of experiments. When the problem of the pump was observed, a smaller orifice (0.25 (0.25 mm) and focusing tube (0.76 mm) were used to be sure that an even lower flow mm) and focusingrate tube could (0.76 not improvemm) were the used situation. to be sure The approximatethat an even maximum lower flow observed rate could pressure not improve thewas situation. 150 MPa The and approximat the focusinge tube maximum diameter observed was 0.76 mmpressure (approximate was 150 diameter MPa of and the focusingimpacting tube diameter jet), and the was yield 0.76 force mm value (approximate was 68 N and diameter the observed of peaksimpacting in Figure jet), 10 are and the yield forcemostly value between was50 and68 N 60 and N (i.e., the between observed 73 and peaks 88% ofin the Figure maximum 10 are value mostly calculated between 50 andfor 60 observed N (i.e., pressure). between Considering 73 and 88% that aof small the amountmaximum of abrasive value was calculated still sucked, for it can be concluded that the measured result correlates with observed physical reality quite well. observed pressure). Considering that a small amount of abrasive was still sucked, it can However, it was just one measurement and immediately after it, the work was stopped to be concluded thatprevent the further measured damage result of the corre machine.lates with observed physical reality quite well. However, it wasSome just new one experiments measuremen have beent and suggested immediately for proving after the it, hinted the work hypotheses was and stopped to preventmapping further the sensordamage behaviour of the machine. properly: Some new• experimentsthe force measurement have been suggested of intentionally for proving made uncut the partshinted (setting hypotheses up the particular and mapping the sensorAWJ behaviour parameters properly: so that the jet has problems with cutting through a sample); • a similar experiment set-up as presented in this article, but with the traverse speeds • the force measurement of intentionally made uncut parts (setting up the particular inducing a significant increase in the signal level (to reduce uncertainty from noise AWJ parametersgenerated so that fromthe jet servomotors); has problems with cutting through a sample); • a similar experiment• the measurement set-up as presented of forces on in machines this article, with but different with the types traverse of engines speeds used for inducing a significantaxes movement; increase in the signal level (to reduce uncertainty from noise generated •fromforce servomotors); measurement showing the influence of the water level in the AWJ attenuator • the measurementonto of signal forces shape on machines and value. with different types of engines used for axes movement;6. Conclusions • force measurementThis paper showing presents theresults influence obtained of the with water a special level force in the sensor AWJ during attenuator AWJ cutting onto signalof shape metals. and The value. sensor measures force signals in three orthogonal axes. The experiments make evident that the online force measurements can be used for the evaluation of the cutting quality provided that the ratio of values is analysed. The results of measurements should also contribute to the understanding of the machining process. The results from the previously presented research were confirmed on a broader scale of materials. These results confirm that for usual conditions, i.e., energy of jet is sufficient for the development of both wear types—cutting and deformation ones, the increase in the traverse speed leads to an increase in the cutting to deformation force ratio up to a certain maximum Materials 2021, 14, 1683 14 of 16

at about half of the limit traverse speed, and then further increase in the traverse speed leads to a decrease in the cutting to deformation force ratio. Measurements prove this presumption very soundly. Nevertheless, some of the measurements were distorted by a higher level of noise. This noise is caused namely by the impact of the rippling water in the waste vessel (jet energy attenuator). Other signals are mechanically interrupted during the cutting process. This problem can be caused by a high level of water in the AWJ energy attenuator, insufﬁcient sample ﬁxing and/or sensor hysteresis. The damaged signals were not included into force analyses, but they are presented as examples and discussed. The detailed investigation of this problem is just in process. Some signal shapes that look different from the typical one indicate a problem with cutting through the material, either due to a pumping machine problem or improper traverse speed setting. These phenomena are to be subjected to a broad intentional investigation in the near future.

Author Contributions: Conceptualization, L.M.H. and D.K.; methodology, L.M.H.; software, M.T.; validation, A.Š., D.K. and P.M.; formal analysis, D.B.; investigation, L.M.H. and P.M.; resources, A.Š. and M.T.; measurement—data acquisition, A.Š., M.T. and D.B.; data processing, M.T. and A.Š.; writing—original draft preparation, A.Š. and L.M.H.; writing—review and editing, L.M.H., D.K. and D.B.; visualization, A.Š.; funding acquisition, L.M.H., D.B. and P.M. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Ministry of Education, Youths and Sports of the Czech Republic, projects numbers SP 018/43, SP 2019/26, SP 2020/45 and SP 2021/64. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: No publicly archived datasets are reported or used. Conﬂicts of Interest: The authors declare no conﬂict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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